The present invention relates to a process for the preparation of finely grained lead azide, in which aqueous solutions of sodium azide and lead nitrate are stirred in a container at a constant temperature, and an apparatus for the preparation of finely grained lead azide according to the invention.
A process for preparation of lead azide is known from the book "Chemistry and Technology of Expolosives" Vol. 3, by T. Urbanski, Pergamon Press 1967, pp. 178, 179. Per batch, lead nitrate in a 9-10% aqueous solution is filled into an open reaction vessel. At a temperature of 50.degree. C., this solution is neutralized with sodium hydroxide until a pH-value of 4.0 is arrived at, as indicated by a color change of methyl orange. A small quantity of dextrine is added, to interfere with the crystal growth of the precipitating lead azide. Then, sodium azide in a 2.7 to 3.0% aqueous solution is added. This solution should be alkaline as indicated by phenolphthalein. Alkalinity is adjusted with a 1N-solution of sulfuric acid. If alkalinity is too low, a calculated quantity of sodium hydroxide is added. This mixture is stirred for an hour at a constant temperature of 50.degree. C. Subsequently, the stirrer is stopped and lead azide precipitates from the solution. The agitator is removed upwardly and the reaction vessel is tilted sideways, so that the lead azide can be filtered off. The lead azide is then washed in water, dried and sieved. The yield of the lead azide produced in this manner is 99%.
The above-described process of preparation of lead azide is so costly because, as is known, with crystalline lead azide, the mere breaking of the crystal needles is liable to trigger an explosion. The necessary additives such as dextrine, polyvinyl alcohol or the like result in the formation of polymorphous lead azide. This has the serious disadvantage of impairing pourability and, thus, dosing of the lead azide in the manufacture of ammunition. What is more, the above-described method of preparation does not fully preclude crystallization of the lead azide, so that a considerable amount of crystal needles with high frictional, impact and temperature sensitivity are still present. The differing particle sizes of the thus produced lead azide also impair fluidity. Another disadvantage of the known process resides in the fact that reaction sets in only after seeding of the solution mixture. Lead content of the waste water amounts to about 13 gr/l, constituting a considerable environmental load.
It is an object of the invention to provide a process and an apparatus for the preparation of finely grained lead azide, wherein the lead azide produced is in the form of a granulate of approximately spherical particles, to obtain better fluidity and less sensitive handling of the lead azide. Also, a reduction of lead-containing waste water is aimed at.
In a process of the preparation of finely grained lead azide, in which aqueous solutions of sodium azide and lead nitrate are continuously stirred in a vessel at constant temperature, this is achieved in that
in a first process step the aqueous solution of lead nitrate and a small amount of an aqueous solution of sodium citrate are poured into the vessel;
in a second process step, the aqueous solution of sodium azide is dosed into the vessel over a period of time of 30 to 40 minutes;
the aqueous solutions poured in the vessel are stirred at a stirring speed of between 0.2 m/sec and 1.0 m/sec, and
the constant temperature is set to a value of 50.degree. C. to 80.degree. C., preferably to 70.degree. C.
The device for the preparation of finely grained lead azide according to this process comprises a cylindrical vessel with a hemispherical bottom and an agitator (also known as stirrer) disk with twisted blades, with the disk being disposed in the lower third of the aqueous solutions filled into the vessel. This largely prevents crystal deposits on the vessel walls.
An important advantage of the invention resides in the fact that the preparation process is significantly simpler and the process steps result in a very uniform and finely grained lead azide crystallization. There is no need to adjust the pH-value of the mixture, as the sodium citrate added acts as a weak buffer. Also, the citrate anion and the lead cation form a weak complex bond, slowing down the reaction between lead and azide. Of particular importance is the fact that the aqueous solution of sodium azide is added during 30 to 40 minutes, preferably 35 minutes, in order to prevent the undesirable formation of undefined crystal agglomerates. As reaction takes place without seeding, this constitutes a further simplification of the process. Because of the higher ratio: azide/lead, the lead content of the waste water is significantly lower than with the conventional process.
Another advantage of the present invention is the fact that now the vessel need no longer be tilted, as the finely grained lead azide is discharged in suspension via a discharge valve in the vessel bottom. This facilitates a substantially more rational and faster production.
A further advantage of the invention pertains to the use of an aqueous solution with 5-7 wt.-%, preferably 6 wt.-% lead nitrate, an aqueous solution with 6-8 wt.-%, preferably 7 wt.-% sodium citrate, and an aqueous solution with 2-4 wt.-%, preferably 3 wt.-% sodium azide. With this composition, it was seen that lead content in the waste water is only about 30% of that resulting from the conventional preparation process.
Experience has shown a stirring speed of 0.5 m/sec to 0.7 m/sec gives good results, producing a very homogeneous granulate with approximately spherical lead azide particles.
In an advantageous design of the apparatus for the preparation of finely grained lead azide, the stirrer blades are set each at an angle of 5.degree. to 15.degree., preferably 10.degree.with the horizontal. This precludes crystallization of the lead azide. At the same time, formation of undesirable agglomerates is prevented by the high stirring velocity of the stirrer.
The inlet nozzles for the dosing of the sodium azide are advantageously arranged in the upper region of the vessel and at an angle to the horizontal of 60.degree. to 80.degree., preferably 70.degree., so that a very uniform and controllable mixing action is ensured.
The discharge valve is advantageously desired as a valve cone movable upwards and downwards on a spindle, which valve cone matches a valve seat inwardly tapering in the vessel bottom.
Other objects and advantages will become apparent in the description below.